Behavior of opendir() after removing a directory with rmdir()

Question

I'm trying to understand how removing the current working directory works from a C program. I'm using the Advanced Programming in Unix Enviroment book as a reference. First, I make the following sequence of calls (/tmp is an existing directory):

//  1. Create a test directory
if (mkdir("/tmp/foo", DIR_PERM) < 0){
    printf("mkdir error\n");
}
else 
    printf("mkdir succeeded\n");

// 2. Change cwd to the test directory
if (chdir("/tmp/foo") < 0)
    printf("chdir error\n");
else
    printf("chdir succeeded\n");

// (2.5) Check the cwd
if (getcwd(cwd, sizeof(cwd)) != NULL)
    printf("Current working directory: %s\n", cwd);
else
    printf("getcwd error\n");

// 3. Remove the test directory
if (rmdir("/tmp/foo") < 0)
    printf("rmdir error\n");
else
    printf("rmdir succeeded\n");

// (3.5) Check current working directory
if (getcwd(cwd, sizeof(cwd)) != NULL)
    printf("Current working directory: %s\n", cwd);
else
    printf("getcwd error\n");

This works as I would expect. mkdir, chdir, and rmdir succeed, getcwd in (2.5) returns the /tmp/foo pathname and getcwd in (3.5) fails.

What I don't understand is the behavior of using opendir afterward on the /tmp/foo directory using ".", "../foo", and "/tmp/foo" as pathnames:

// 4. Open the removed directory using "." 
if ((dp = opendir(".")) == NULL)
    printf("1. opendir error\n");
else{
    printf("1. opendir succeeded\n");
    while((dirp = readdir(dp)) != NULL)
        printf("%s\n", dirp->d_name);
}

// 5. Open the removed directory using "../foo"
if ((dp = opendir("../foo")) == NULL)
    printf("2. opendir error\n");
else
    printf("2. opendir succeeded\n");

// 6. Open the removed directory using "/tmp/foo"
if ((dp=opendir("/tmp/foo")) == NULL)
    printf("3. error\n");
else
    printf("3. opendir succeeded\n");

The output of the above code snippet is

1. opendir succeeded
2. opendir error
3. opendir error

Why does opendir(".") work but opendir("../foo") doesn't? As I understand it, the link count of the /tmp/foo directory becomes 0 after rmdir. The dot and dot-dot entries are removed before that function returns and reading the directory in 4. after the successful opendir call doesn't print anything. How does opendir know what dot stands for if that entry was removed during rmdir? And why wouldn't it resolve dot-dot if it can resolve dot?

Answer 1

Why does opendir(".") work but opendir("../foo") doesn't?

the link count of the /tmp/foo directory becomes 0 after rmdir

No, your program's current working directory still points there. It's like an open inode.

$ mkdir -p /tmp/foo/abc
$ cd /tmp/foo
$ ls -la /proc/self/cwd
lrwxrwxrwx 1 kamil kamil 0 mar 22 18:35 /proc/self/cwd -> /tmp/foo/
$ rmdir /tmp/foo
$ ls -la /proc/self/cwd
lrwxrwxrwx 1 kamil kamil 0 mar 22 18:36 /proc/self/cwd -> '/tmp/foo (deleted)'

How does opendir know what dot stands for if that entry was removed during rmdir?

Kernel keeps track of processes and keeps track of processes current working directories and it knows what is ..

why wouldn't it resolve dot-dot if it can resolve dot?

It resolves ... It does not resolve ../foo, there is no foo at .. after it was deleted. opendir('..') should still work.